Bidirectional, co-located laser and detector

ABSTRACT

Bi-directional optical signal transmission apparatus and methods are disclosed. In one embodiment, an optical detector is configured to receive an optical signal from an end of an optical fiber. An optical element is located at the detector surface and is configured to receive and direct a laser beam. A laser beam is directed by the optical element through the end into the optical fiber substantially parallel with the principal axis.

FIELD OF THE INVENTION

This invention relates generally to optical transmission along fiberoptic conduits and, more specifically, to optical data transmission.

BACKGROUND OF THE INVENTION

Most modern point-to-point optical communication links require thatnodes communicate in both directions, as shown in FIG. 1 a. In thisexample, a bidirectional communication installation 12 facilitatesbidirectional communication between a first transceiver 21 a and asecond transceiver 21 b. A first discrete optical fiber 36 a carries asignal from a first transmitter 24 a to a first receiver 27 a, and asecond discrete optical fiber 36 b carries a signal in the oppositedirection from a second transmitter 24 b to a second receiver 27 b. Tocommunicate between the first transceiver 21 a and the secondtransceiver 21 b, each of the optical cables 36 a, 36 b require opticalconnectors 42 for each bulkhead 45 they pass through.

In general, optical connectors 42, contacts within the connectors, andcable represent a substantial part of the cost of a fiber optic link(typically greater than 50% of link cost). They also constitute theleast reliable components within the link, in part because connectorsare intended to be de-mated, allowing contamination, and in part,because these components (in particular the cables) are exposed tomaintenance-induced failure due to their accessibility within equipmentbays, etc. Thus, any scheme that allows both signal paths, the incomingsignal 33 and the outgoing signal 30 (FIG. 1 a), to be combined over asingle fiber substantially reduces cost and improves reliability (e.g.there are half as many connectors and half as many cable segments to gobad). There are several techniques that have been used to accomplishthis bidirectional transmission, each with disadvantages.

A first alternate installation scheme 15 is to use an optical coupler 36f to combine the signals 30, 33 on a single optical fiber 36 strungbetween the first transceiver 21 a and the second transceiver 21 b, asshown in FIG. 1 b. The optical coupler 36 f in the first alternateinstallation scheme 15 may be a fused bi-conic tapered coupler with thecores of single optical fiber 36 and the second optical fiber 36 d inintimate contact with one another. One object of the first alternateinstallation 15 is accomplished in that the number of optical connectors42 through the bulkheads 45 is decreased by a factor of two.

One disadvantage of this first alternate installation 15 is that half ofthe light from the transmitter (assuming a 50–50 coupler) is lost beforeit makes its way to the single optical fiber 36 through the bulkheads 45at the connectors 42 because half the light is coupled into an unusedfourth port 36 e. There is a further loss at the other receiving end ofthe single optical fiber 36, where half the light is coupled into thetransmitter 24 b rather than the receiver 27 a. As a result, atheoretically perfect link will exhibit a 6dB loss from transmitter 24 ato receiver 27 a just because of these couplers 36 f. It is desirable inmost links to avoid this very large loss.

A second alternate installation 18 includes use of two discretewavelengths 51, 54, as shown in FIG. 1 c. The use of dichroic(two-color) filters 57 a, 57 b (sometimes called dichroic mirrors) isjust one of many possible approaches but is the most straightforwardschematically and is used here to describe the general approach. Thetransmitter 24 a at the first transceiver 21 a creates light 51 withwavelength of λ₁. The filter at this node transmits light 51 at λ₁ butreflects light 54 at λ₂. Incoming light 54 carrying an incoming signal33 with wavelength λ₂ is routed to the receiver 27 b along the fiber 37.Similarly, the right-hand node dichroic filter 57 b transmits light 54at wavelength λ₂ carrying an outgoing signal 30 but reflects light 51 atwavelength λ₁. Alternatively, fibers 30 and 37 may be replaced by freespace illumination from the transmitter 24 a and to the receiver 27 bwithout affecting this example.

There are several disadvantages to the second alternate installation 18.Principal among these is the fact that light 54 returning fromreflections on the single optical fiber 36 is transmitted entirely backto the transmitter 24 b of origin. This may be disadvantageous as it cancause laser instability. In addition, the transceivers 21 a, 21 b at thetwo ends of the link are unique from one another. In FIG. 1 c, thetransmitter 24 a on the left-hand side emits light 51 at wavelength λ₁but the transmitter 24 b on the right-hand side emits light 54 atwavelength λ₂. Likewise, the receivers 27 a, 27 b operate at differentwavelengths. Finally, as systems increasingly make use of multiplewavelengths, it is important to keep in mind that this scheme halves thenumber of wavelengths available for information carrying. Notably,although this approach is conceptually simple, dichroic filters 57 a, 57b are costly and that cost may drive the cost of the installation.

Therefore, a need exists for bidirectional optical signal transmissionapparatus and methods that at least partially mitigate the above-noteddisadvantages in an economical manner.

SUMMARY OF THE INVENTION

The present invention is directed to bi-directional optical signaltransmission apparatus and methods. Embodiments of bi-directionaloptical signal transmission apparatus and methods in accordance with thepresent invention may advantageously reduce the number of connectionsand the length of fiber by half, reduce the installation cost of fiberoptic cable plant by half, provide for automatic diagnostics in the formof an In-Service Optical Time Domain Reflectometer, and provide thesecapabilities while allowing simultaneous communication in bothdirections with the same wavelength of light.

In one embodiment, an optical detector in accordance with the inventionis configured to receive an optical signal from an end of an opticalfiber and defines an optical element. The end has a principal axis and aradius. The optical element is located within the radius and configuredto receive and direct a laser beam. A laser beam is directed through theoptical element into the end into the optical fiber substantiallyparallel with the principal axis. The optical signal detector may belocated at an end of an optical fiber, thereby enabling bidirectionalsignal communication over the fiber.

It will be appreciated that the features, functions, and advantages canbe achieved independently in various embodiments of the presentinvention, or may be combined in yet other embodiments, as describedmore fully below.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred and alternative embodiments of the present invention aredescribed in detail below with reference to the following drawings.

FIG. 1 a is a schematic diagram of a prior art bidirectional fiberoptical cable installation;

FIG. 1 b is a schematic diagram of a first alternative prior artbidirectional fiber optical cable installation;

FIG. 1 c is a schematic diagram of a second alternative prior artbidirectional fiber optical cable installation;

FIG. 2 is a cut-away diagram of an optical system including a detectorhaving an optical port on a major axis of an optical fiber in accordancewith an embodiment of the invention;

FIG. 3 is a cut-away diagram of an optical system including a detectorhaving an optical port off the major axis of the optical fiber inaccordance with another embodiment of the invention;

FIG. 4 a is a cut-away diagram of an optical system including anobliquely illuminated detector incorporating a smaller mirrored portionoff the major axis of the optical fiber in accordance with yet anotherembodiment of the invention;

FIG. 4 b is a cut-away diagram of an optical system including a detectorincorporating an angled mirror off the major axis of the optical fiberin accordance with still another embodiment of the invention;

FIG. 5 is a flow chart of a method of bidirectional optical signaltransmission in accordance with an embodiment of the invention; and

FIG. 6 is a side elevational view of an aircraft 600 having one or moreoptical systems in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to bi-directional optical signaltransmission apparatus and methods. Many specific details of certainembodiments of the invention are set forth in the following descriptionand in FIGS. 2–6 to provide a thorough understanding of suchembodiments. One skilled in the art, however, will understand that thepresent invention may have additional embodiments, or that the presentinvention may be practiced without several of the details described inthe following description.

By way of overview, in one embodiment, an optical detector is configuredto receive an optical signal from an end of an optical fiber and definesan optical element. The end has a principal axis and a radius. Theoptical element is located within the radius and configured to receiveand direct a laser beam. A laser beam is directed through the end of theoptical element and into the optical fiber substantially parallel withthe principal axis.

FIG. 2 is a schematic of an optical system 200 that includes an opticalsignal detector 63 in accordance with an embodiment of the presentinvention. The optical system 200 includes a laser 24 operativelypositioned on a first side of the optical signal detector 63, and amultimode fiber 36 positioned on a second side of the optical signaldetector 63. In this embodiment, the optical signal detector 63 definesan optical port 66. As shown in FIG. 2, a laser beam 99 generated by thelaser 24 passes through the optical port 66 in the optical signaldetector 63 en route to the multimode fiber 36. As described more fullybelow, the optical signal detector 63 advantageously detects an incomingsignal 33, and may also allow capture of approximately all of the laserbeam 99 (save for a small Fresnel reflection that always occurs atlaser/fiber interface, unless minimized by well-known means). Thus, theinventive optical signal detector 63 may provide an insertion losspenalty that may be nearly zero.

Modem semiconductor lasers can launch their power into areas withdiameters smaller than 10 μm. Because the light traveling from a 62.5μm-core multimode fiber has a waist that is approximately 62.5 μm,allowing the optical port 66 to be relatively small (for example,diameter 20 μm), and thus, will not significantly compromise theperformance of the detector 63. Outgoing signals 30 pass out of thelaser 24 and into the optical fiber 36, while incoming signals 33 passout of the optical fiber 36 and are detected at the detector 63.

With continued reference to FIG. 2, for the light carrying the incomingsignal 33 from the fiber 36 that is detected at the detector 63, thereis a loss of energy due to the area of port 66 and its placement at thedetector 63 center. If the light energy were uniformly distributedacross the surface of an end 39 of the fiber 36 such that the lightenergy shining on the detector 63 was uniform across the surface, and ifthe port 66 is configured with a 20 μm diameter, with the fiber 36having a diameter of approximately 62.5 μm, then the loss that willoccur due to light going back through the port 66 and missing thedetector 63 is given by:

$\begin{matrix}{{{loss}\mspace{14mu}({dB})} = {{10{\log\left( {1 - \frac{\pi\; R_{1}^{2}}{\pi\; R_{2}^{2}}} \right)}} = {{10{\log\left\lbrack {1 - \left( \frac{20}{62.5} \right)^{2}} \right\rbrack}} = {0.5\mspace{14mu}{dB}}}}} & (1)\end{matrix}$

where R1 is the hole diameter and R2 is the incoming beam waist diameteror optimally, the diameter of the fiber 36.

In practice, however, light energy distributions across the face of amultimode fiber 36 are rarely uniform, particularly when the source is alaser transmitter 24 or a graded index fiber. Instead, the light energytends to be distributed in a manner that concentrates the lightintensity at the center of the fiber 36. A Gaussian distribution is acloser approximation of the distribution of light energy across thesurface of the end 39. Assuming a Gaussian distribution, placing theport 66 at the center of the detector 63, coinciding with the center ofthe end 39 and the maxima of energy distribution, increases the loss dueto the presence of the port 66. The approximate energy loss to thedetector 63 may be on the order of 1 dB.

FIG. 3 is a cut-away diagram of an optical system 300 including adetector 63 having an optical port 66 off the major axis of the opticalfiber 36 in accordance with another embodiment of the invention. Becausethe port 66 is offset from the center of the detector 63, the loss oflight energy from the fiber 36 to the detector 63 is greatly reduced,improving reception of the incoming signal 33 without compromising theability to receive the whole of the laser beam 99 from the transmitter24 carrying the outgoing signal 30.

The performance of the detector 63 defining a port 66 may be adverselyimpacted by additional dark current. Thermal dark current in the absenceof light degrades most light-sensitive detectors 63 and would increasebecause of surface states caused by the presence of the aperture withinthe sensitive region, so an aperture is a good example but not a goodidea.

FIG. 4 a shows an optical system 400 including an elliptical-shapeddetector 63 placed at an oblique angle to the beam from the end 39 of afiber 36.

Note that FIGS. 3 and 4 show only the core of the optical fiber.

In this embodiment, a transmitter 24 shines a laser beam 99 onto themirror 60 to reflect the beam 30 into the end of the optical fiber 36.The light energy in the incoming signal 33 is spread across a greatersurface area of the detector 63 resulting in a nearly total absorptionof energy except for the received energy hitting the mirror 60, which iswasted. Rather than a port 66, the laser beam 99 from the transmitter 24is trained on a mirror 60 on the surface of the detector 63.Advantageously, the mirror 60 may readily be formed on the detector 63by evaporating a metallic surface overlaying the semiconductor substratemaking up the detector 63, and then masking the surface during etching,though other alternative means will also serve.

FIG. 4 b shows another embodiment of an optical system 450 that alsouses a mirror 60 mounted on the detector 63 to redirect a laser beam 99into an optical fiber 36.

FIG. 5 is a flow chart of a method 70 of bidirectional optical signaltransmission in accordance with an embodiment of the invention. Asdescribed above with reference to FIGS. 2, 3, 4 a, and 4 b, the method70 includes providing an optical detector 63 at a block 72. The opticaldetector 63 is configured to receive an optical signal 33 emitted froman end 39 of the optical fiber 36, and also defines an optical element66 or 60 in the detector 63 to admit or to direct a laser beam 99. Theoptical element might be a simple port that allows optical signals topass through the detector (e.g. FIGS. 2–3), or may be any other suitableoptical element, such as a mirror, a bifurcating mirror, a prism, alens, a diffraction grating, an etalon, or a Brewster plate, so long asit may be positioned to direct the laser beam 99 into the end 39 of theoptical fiber 36 along the principal axis of the fiber 36.

As discussed above, the optical element 66 or 60 may be located topropagate a laser beam 99 efficiently into an end 39 while minimallydetracting from the efficiency of the detector 63 by occluding as littleof the laser beam 99 as it leaves the optical fiber 36.

At a block 75, the laser beam 99 may be directed through the opticalelement 66 or 60 through the end 39 of the optical fiber 36substantially parallel with the principal axis. The resulting outgoingsignal 33 may then pass along the same path as the incoming signal 30,enabling bidirectional communication.

Embodiments of bidirectional optical signal transmission apparatus andmethods in accordance with the present invention may provide significantadvantages over prior art apparatus and methods. For example, the numberof connections and the length of fiber may be reduced by up to one half,causing a corresponding reduction in the installation cost of fiberoptic cable plant. Similarly, embodiments of apparatus and methods inaccordance with the present invention may advantageously allow forautomatic diagnostics in the form of fiber optic links having a built-intest as disclosed, for example, in co-pending, commonly-owned U.S.patent application Ser. No. 10/644,124 filed on Aug. 20, 2003, or anin-service optical time domain reflectometer as disclosed, for example,in co-pending, commonly-owned U.S. Patent Application (undetermined),filed under on (undetermined), which applications are incorporatedherein by reference. Finally, apparatus and methods in accordance withthe present invention may provide these capabilities while allowingsimultaneous communication in both directions with the same wavelengthof light.

It will be appreciated that a wide variety of apparatus may be conceivedthat incorporate optical systems that include apparatus and methods inaccordance with various embodiments of the present invention. Forexample, FIG. 6 is a side elevational view of an aircraft 600 having oneor more optical systems 602 in accordance with embodiments of thepresent invention. In general, except for the various optical systems602, the various components and subsystems of the aircraft 600 may be ofknown construction and, for the sake of brevity, will not be describedin detail herein.

As shown in FIG. 6, the aircraft 600 includes one or more propulsionunits 604 coupled to a fuselage 605, wing assemblies 606 (or otherlifting surfaces), a tail assembly 608, a landing assembly 610, acontrol system 612 (not visible), and a host of other systems andsubsystems that enable proper operation of the aircraft 600. Theaircraft 600 shown in FIG. 6 is generally representative of a commercialpassenger aircraft, including, for example, the 737, 747, 757, 767, and777 models commercially-available from The Boeing Company of Chicago,Ill. The inventive apparatus and methods disclosed herein, however, mayalso be employed in any virtually any other types of aircraft. Forexample, the teachings of the present invention may be utilized in othertypes of passenger aircraft, fighter aircraft, cargo aircraft, rotaryaircraft, and any other types of manned or unmanned aircraft, includingthose described, for example, in The Illustrated Encyclopedia ofMilitary Aircraft by Enzo Angelucci, published by Book Sales Publishers,September 2001, and in Jane's All the World's Aircraft published byJane's Information Group of Coulsdon, Surrey, United Kingdom, whichtexts are incorporated herein by reference.

More specifically, the aircraft 600 may include one or more embodimentsof optical systems 602 a in accordance with the present inventionincorporated into the flight control system 612, or into optical controlsystems 602 c for controlling the propulsion units 604, including, forexample and not by way of limitation, the optical systems generallydisclosed in U.S. Pat. No. 5,809,220 issued to Morrison et al., U.S.Pat. No. 6,369,897 B1 issued to Rice et al., U.S. Pat. No. 6,266,169 B1issued to Tomooka et al., U.S. Pat. No. 5,653,174 issued to Halus, U.S.Pat. No. 5,295,212 issued to Morton et al., U.S. Pat. No. 5,222,166issued to Weltha, and U.S. Pat. No. 5,119,679 issued to Frisch. Clearly,a wide variety of optical systems 602 in accordance with embodiments ofthe present invention may be conceived for incorporation into thevarious subsystems of the aircraft 600.

While the preferred embodiment of the invention has been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

1. A method for bidirectional optical signal transmission through anoptical fiber having a principal axis, the method comprising: providingan optical detector in optical communication with an end of the opticalfiber, the optical detector being configured to receive an opticalsignal from the end of the optical fiber and defining an opticalelement, the optical element being located at least partially within aradius of the end and configured to receive and direct an optical energyand defining a mirror disposed on a surface of the optical detector; anddirecting the optical energy onto the optical element and toward the endof the optical fiber substantially parallel with the principal axis ofthe optical fiber.
 2. The method of claim 1, wherein providing anoptical detector includes providing an optical detector engaged with anend of the optical fiber.
 3. The method of claim 1, wherein providing anoptical detector defining an optical element includes providing anoptical detector defining an optical port disposed therethrough.
 4. Themethod of claim 1, wherein directing the optical energy onto the opticalelement includes directing a laser beam onto the optical element.
 5. Themethod of claim 1, wherein providing an optical detector defining anoptical element includes providing an optical detector defining anoptical port disposed therethrough, and wherein directing the opticalenergy onto the optical element includes directing the optical energythrough the optical port.
 6. The method of claim 1, wherein providing anoptical detector in optical communication with an end of the opticalfiber and defining an optical element includes providing an opticaldetector in communication with the end defining a mirror disposed on asurface of the optical detector.
 7. The method of claim 1, whereinproviding an optical detector in optical communication with an end ofthe optical fiber and defining an optical element includes providing anoptical detector spaced apart from an end of the optical fiber.
 8. Themethod of claim 1, wherein providing an optical detector defining anoptical element includes providing an optical detector defining anoptical element disposed at an intersection of the principal axis withthe detector.
 9. The method of claim 1, wherein the detector defines theoptical element, such that the principal axis does not intersect theoptical element.
 10. The method of claim 1, wherein the optical elementincludes at least one of a diffraction grating, a Brewster plate, anetalon, a prism, and a lens.
 11. An optical component, comprising: adetector adapted to be positioned in optical communication with an endof an optical fiber and configured to receive an optical signal from theoptical fiber and to generate an electronic signal responsive to theoptical signal, the detector defining an optical element positionable atleast partially within a radius of the end of the optical fiber, theoptical element being configured to receive and direct an optical energytoward the end of the optical fiber and defining a mirror disposed on asurface of the optical detector.
 12. The component of claim 11, whereinthe detector defines the optical element at an intersection of theprincipal axis with the detector.
 13. The component of claim 11, whereinthe detector is further adapted to communicate optically with the end ofthe optical fiber.
 14. The component of claim 11, wherein the opticalelement comprises an optical port disposed through the detector.
 15. Thecomponent of claim 11, wherein the optical element is configured toreceive and direct a laser beam toward the end of the optical fiber. 16.The component of claim 11, wherein the detector is adapted to be engagedwith an end of the optical fiber.
 17. The component of claim 11, whereinthe detector is adapted to be spaced apart from an end of the opticalfiber.
 18. The component of claim 11, wherein the optical elementincludes at least one of a mirror, a diffraction grating, a Brewsterplate, an etalon, a prism, and a lens.
 19. The component of claim 11,further comprising a transmitter operatively positioned proximate thedetector and adapted to transmit the optical energy onto the opticalelement.
 20. The component of claim 11, wherein the transmitter includesa laser.
 21. A fiber optic system for bidirectional optical signaltransmission, comprising: an optical fiber including at least one end,the end having a radius through which light passes and a principal axis;a detector adapted to be positioned at least proximate to the end of theoptical, fiber and configured to receive an optical signal from theoptical fiber and to generate an electronic signal responsive to theoptical signal, the detector defining an optical element positionable atleast partially within a radius of the end of the optical fiber, theoptical element being configured to receive and direct an optical energytoward the end of the optical fiber and defining a mirror disposed on asurface of the optical detector; and a transmitter operativelypositioned proximate the detector and adapted to transmit the opticalenergy onto the optical element.
 22. The system of claim 21, wherein thedetector defines the optical element at an intersection of the principalaxis with the detector.
 23. The system of claim 21, wherein the detectoris further adapted to be engaged into contact with the end of theoptical fiber.
 24. The system of claim 21, wherein the optical elementcomprises an optical port disposed through the detector.
 25. The systemof claim 21, wherein the optical element is configured to receive anddirect a laser beam toward the end of the optical fiber.
 26. The systemof claim 21, wherein the detector is adapted to be engaged proximal tothe optical fiber.
 27. The system of claim 21, wherein the detector isadapted to be spaced apart from an end of the optical fiber.
 28. Thesystem of claim 21, wherein the optical element includes at least one ofa diffraction grating, a Brewster plate, an etalon, a prism, and a lens.29. The system of claim 21, wherein the transmitter includes a laser.30. An aerospace vehicle, comprising: a fuselage; a propulsion systemoperatively coupled to the fuselage; and an optical system operativelydisposed at least partially within the fuselage, the optical systemcomprising: an optical fiber including at least one end, the end havinga radius and a principal axis; a detector adapted to be positioned atleast proximate to the end of the optical fiber and configured toreceive an optical signal from the optical fiber and to generate anelectronic signal responsive to the optical signal, the detectordefining an optical element positionable at least partially within aradius of the end of the optical fiber, the optical element beingconfigured to receive and direct an optical energy toward the end of theoptical fiber and defining a mirror disposed on a surface of the opticaldetector; and a transmitter operatively positioned proximate thedetector and adapted to transmit the optical energy onto the opticalelement.
 31. The vehicle of claim 30, wherein the detector defines theoptical element at an intersection of the principal axis with thedetector.
 32. The vehicle of claim 30, wherein the detector is furtheradapted to be proximal to the end of the optical fiber.
 33. The vehicleof claim 30, wherein the optical element comprises an optical portdisposed through the detector.
 34. The vehicle of claim 30, wherein thedetector is adapted to be spaced apart from an end of the optical fiber.35. The vehicle of claim 30, wherein the optical element includes atleast one of a mirror, a diffraction grating, a Brewster plate, anetalon, a prism, and a lens.
 36. The vehicle of claim 30, wherein thetransmitter includes a laser.